1. Field of the Disclosure
The present disclosure relates to a driving support device for a vehicle that carries out a lane departure prevention support control for supporting driving by a driver so that the vehicle travels in a lane.
2. Description of Related Art
A driving support device for a vehicle that carries out a lane departure prevention support control has been known. In the lane departure prevention support control, white lines on the right and left of a road are detected by, for example, a camera sensor. A target trace line in a traveling lane such as a center line between the right and left white lines is set based on these right and left white lines. Then, a driver's steering operation is supported by a steering auxiliary torque being given to a steering mechanism so that a traveling position of a host vehicle is maintained in the vicinity of the target trace line. Also known are types of lane departure prevention support control calling the driver's steering operation by allowing a buzzer to sound and giving the steering mechanism a small steering auxiliary torque when the host vehicle is likely to depart from the traveling lane. The former lane departure prevention support control is referred to as a lane keeping support control, and the latter lane departure prevention support control is referred to as a lane departure alarm control.
This lane departure prevention support control is different from an autonomous driving control, is to support the driver's steering wheel operation so that the host vehicle does not depart from the target trace line or the traveling lane, and is not a control implying that the driver's steering wheel operation is unnecessary. A driving support device that is disclosed in Japanese Patent Application Publication No. 2014-142965 is configured to interrupt the lane departure prevention support control in a case where the driver's hand-off driving is detected while the lane departure prevention support control is carried out. The detection of the driver's hand-off driving is determined when, for example, the steering wheel operation does not continue to be performed for at least a predetermined period of time.
In general, the determination of the presence or absence of the steering wheel operation is performed based on a steering torque that is detected by a torque sensor which is disposed in a steering shaft. It is determined that the steering wheel operation is not performed when the steering torque is less than a threshold. The driver's hand-off driving is determined when a duration in which the steering torque is less than the threshold continues for at least a predetermined period of time.
A vehicle speed region in which the lane departure prevention support control is carried out is limited so that the lane departure prevention support control is carried out only during a high-speed traveling. In this regard, there is a demand for carrying out the lane departure prevention support control in all vehicle speed regions with the vehicle speed region expanded to a low-speed side. A response to this demand is likely to result in a more-than-necessary interruption of the lane departure prevention support control that is attributable to the determination of the driver's hand-off driving during a low-speed traveling, which might cause the driver to be bothered.
During the low-speed traveling, for example, there is a case where the vehicle is allowed to travel along the traveling lane even without the driver's steering wheel operation. In this case, the duration in which the steering operation is not performed might reach a predetermined length of time despite the steering wheel being gripped by the driver. In this case, the driver's hand-off driving is determined and the lane departure prevention support control is interrupted. Accordingly, the driver might be given no driving support during the low-speed traveling despite the driver performing a normal driving operation.